In deepwater ocean bottom surveys, autonomous seismic detectors are placed on the seabed and make continuous or intermittent seismic recordings. These detectors may be referred to as ocean bottom seismometers (OBS). During the recording periods, a plurality of seismic signals (“shots”) may be transmitted into the ocean and/or into the seafloor for the purpose of seismically illuminating the subsurface formations, as illustrated schematically in FIG. 1. Seismic signals that have been refracted, reflected and/or transmitted through the subsurface are recorded by the seismic detectors. After recording seismic signals for a period of time, the nodes are recovered and the seismic data they have recorded are processed for the purpose of extracting information about the subsurface. In order to extract meaningful information, it is advantageous to determine the location and relative timing of each seismic node.
The very precise clocks that are required for accurate seismic data analyses tend to drift unpredictably over time. Some clocks may lose time while others may gain it, and the rate of drift for a given clock may vary over time. Over the two-month time period of a typical seismic survey, clocks may easily drift as much as 50 ms in either direction. Finally, operator errors in the initial clock synchronization or final data collection may add to the timing-derived errors.
For these reasons, there remains a need for improving the degree of precision that is possible in ocean bottom seismic surveys.